Multisheet superplastically formed, diffusion bonded, expanded metal sandwich structures have been in use for many years, primarily in the aerospace industry, because of the low cost, high temperature capability and good strength and stiffness per unit weight that these structures offer. Various processes for fabricating these structures have been developed in the past, with various degrees of success, but all have proven costly and slow to produce, and often they have been prone to produce defective or unreliable parts.
Most of the existing techniques for fabricating such structures, including the truss core technique shown in U.S. Pat. No. 3,927,817 to Hamilton, utilize superplastic forming of a stack of sheets in a die having a cavity shaped like the final sandwich structure. The stack includes one or more core sheets that are selectively joined to each other, when there is more than one core sheet, and to a top and bottom sheet that form the top and bottom outside skins of the sandwich structure. The stack is inflated at superplastic temperature with gas pressure to expand the top and bottom sheets outwardly against the interior walls of the die cavity with gas pressure in a die cavity to the desired exterior dimensions. During superplastic forming, the core sheets stretch between their attachment areas to the top and bottom skins as those skins expand toward the boundary surfaces of the die cavity.
Early developments of techniques for fabricating multi-sheet expanded metal sandwich structures utilized diffusion bonding to join the core sheets along selective areas to produce the desired core structure. These techniques require the accurate placement of stop-off to prevent diffusion bonding in areas where adjacent sheets were not intended to bond together. Diffusion bonding is a desirable joining method because the junction retains superplastic qualities, but it has been difficult to produce a clean junction line free of stop-off that is narrow enough, and diffusion bonding can be a lengthy process with long holding times in the press at elevated temperatures, preventing the press from being used for other production. The capital intensive and time consuming nature of the diffusion bonding process lead to research into other techniques for joining the core sheets of multisheet stack that would be faster, more reliable, and less costly.
Another joining method, shown in U.S. Pat. Nos. 4,217,397 and 4,304,821 to Hayase et al., uses resistance welding of the core sheets along the selected lines to establish the junction lines between the core sheets, leaving gaps in the weld lines for passage of forming gas into the cells. This process was faster than the diffusion bonding technique, but still required that the core and face sheets not be loaded into a hot die to avoid premature diffusion bonding of the core sheets to each other. After closing the die, the stack could be purged and pressurized to slightly inflate the stack and separated the sheets from one another so that they would not diffusion bond together where no bonding was desired. The die would then be heated to superplastic temperature and forming gas would be admitted under pressure into the stack to superplastically expand the top and bottom sheets against the walls of the die cavity and stretch the core sheets between the top and bottom sheets to form the desired sandwich structure.
To prevent premature diffusion bonding of the face sheets in the stack with the core sheets, a device is used in the apparatus of the Hayase et al. patents to hold the face sheets spaced apart from the core sheets. Eight separate tooling pieces are shown for this purpose, which increases the cost and complexity of the forming process. For a high rate production operation, it would be preferable to simplify the tooling and enable the parts to be loaded into the die while it is hot, to achieve an increased production rate and lower production cost.
For successful forming to occur, a pressure differential must be established between the face sheet zones and the core sheet zones, and this pressure differential must be equalized over both face sheet zones. Otherwise, the core sheets will form unevenly and will result in excessive thinning.
Heating titanium to a high temperature in the presence of oxygen creates a surface layer of alpha case which is a hard but very brittle composition and is unacceptable in structural parts because of its tendency to crack. Such cracks could grow in a fatigue environment and lead to failure of the part. Consequently, it is desirable to purge oxygen and moisture from the stack of sheets of before heating to elevated temperatures. An ideal process would be one in which the stack of sheets is sealed and purged of oxygen and moisture before loading so the sealed pack could be loaded into a hot die without the danger of alpha case forming before the stack is purged and without using expensive press time to purge the stack and then slowly bring the die up to superplastic temperature.
Another fastening technique, shown in U.S. Pat. No. 4,603,089 to Bampton, uses a CO.sub.2 laser to weld sheets in the stack together. However, the Bampton disclosure does not teach any way to hold the sheets together while they are being laser welded, and indeed does not disclose any apparatus at all to perform the welding operation. In fact, in a production operation for making a laser-welded multisheet expanded metal sandwich structure, such as that shown in U.S. Pat. No. 5,330,092 to Gregg et al., it is necessary to press the sheets into intimate contact to obtain a quality weld, and to do so with a high speed, efficient, high production rate apparatus in order to benefit from the potential benefits that laser welding has to offer. In addition to exerting a press-up force on the sheets during welding, such an apparatus ideally would protect the weld area from oxidation at high temperature that occurs during laser welding of titanium.
Weld cratering and tight radii at the start and stop of the weld are inherent limitations of laser welding. They are the consequences of the high intensity, narrowly focused nature of the beam, and have in the past resulted in sharp termination points that concentrated stresses at those points which can rip the core sheet when the core is pressurized by forming gas during superplastic forming. The laser naturally produces a "keyhole" weldment that forms a crater at the weld termination, severely undercutting the top sheet at the end point of a stitch weld. Such welds weaken the top sheet of the core stack at the weld termination at a point that experiences high stress during inflation by gas pressure during superplastic forming. A production process that optimally utilizes the potential benefits of laser welding would eliminate these weak points at the beginning and terminating ends of the weld.